In some approaches, planar magnetic acoustic transducers use a flat, lightweight diaphragm suspended in a magnetic field, rather than a cone attached to a voice coil. The diaphragm in a planar magnetic transducer has a conductive circuit pattern that, when energized with an electric current, reacts with the magnetic field to create forces that move the diaphragm to produce sound.
Diaphragm material consists of a very thin, flexible, and durable substrate. One example material suitable for this purpose is Kapton® polyimide film, as manufactured and marketed by DuPont™ of Research Triangle Park, N.C. The substrate is provided with a thin layer of electrically conductive material that is either laminated to or deposited on one or both faces of the substrate. Thus, diaphragms most commonly comprise either two layers: the conductive, often metal, layer and the substrate; or three layers: the conductive layer, an adhesive layer, and the substrate layer. If both sides of the substrate are to have a conductive layer, this represents an additional layer, or two layers if there is an adhesive layer between the substrate layer and the conductive layer. The conductive layer (or layers) is etched or otherwise cut to produce the conductive circuit pattern, either before or after being attached to the substrate.
The magnetic field is typically produced by a planar array of bar magnets, the bar magnets spaced apart regularly, but aligned parallel to each other, the poles of the bar magnets oriented to be perpendicular to the layer the magnets form. The diaphragm is suspended above the magnets, and substantial portions of the electrically conductive circuit pattern run parallel to individual bar magnets, as when current passes through these portions of the circuit, an induced magnetic field will react with the field produced by the magnets, causing the conductor, and the attached diaphragm, to be drawn to or away from the magnets.
However, there are drawbacks to this classic planar magnetic acoustic transducer design. The electrically conductive pattern can only handle so much power without having to increase the amount of conductive material, which alters the frequency response of the diaphragm due to increased mass and stiffness of the conductive material. This places a limit on the amount of acoustic power that can be developed by a diaphragm. Additional limitations of this design include non-linearity caused by variations in magnetic flux density between individual magnets, and variations with distance from the magnets. Another limitation is that combinations of audio signals from different sources must be electrically mixed before being used to drive the single diaphragm through a single electrically conductive circuit pattern, or if multiple patterns are used, then current capacity of one has been sacrificed for the other. Still another limitation is that when such signals are mixed and provided to a common transducer (the diaphragm), they are both subject to that transducer's resonances and other responses, which may not be optimal for one signal or the other, requiring additional power and equalization to obtain a desired result.
Another drawback of the classic design is when used in noise cancellation systems, where a separate microphone, near the edge of a planar magnetic transducer, is used to detect noise, which is then to be cancelled for a listener by a conjugate signal being fed to the transducer. In such an embodiment, the position of the microphone is not well matched to the natural resonances and other tunings of the transducer, nor are the axes of the microphone and transducer well aligned, for the purpose of addressing noise coming from different directions equally well. If the diaphragm is used as both a microphonic detector (the input transducer) and as a speaker (the output transducer), whether through separate electrically conductive circuits or a common one, there are significant limitations in differentiating what portion of the input signal is the result of noise that should be cancelled, and what portion is induced by the output signal and non-linearity of the diaphragm and magnetic fields.